1. Field of the Invention
This invention relates generally to semiconductor processing equipment and more particularly to semiconductor wafer cassettes used in conjunction with such equipment.
2. Description of the Related Art
In semiconductor processing equipment, such as ion implanters, wafers are often loaded and unloaded from cassettes having parallel, vertical slots each of which is receptive to a single, vertically oriented wafer. These cassettes may be mounted, for example, on cassettes trays coupled to the paddles of a load lock carousel. Wafers may be selectively lifted from the cassette by means of a vertical transport mechanism for transfer into the processing equipment.
In prior art processing equipment (see, for example, U.S. Pat. No. 4,776,744 of Paul Stonestreet et al., entitled "Systems and Methods for Wafer Handling in Semiconductor Process Equipment") the accurate manufacture of cassettes has been relied upon to assure that wafers were precisely situated within the cassette so as to allow accurate selection of wafers out of the cassette by automated equipment. However, in practice, cassettes manufactured from molded plastic have often failed to provide the necessary precise positioning of wafers required. Furthermore, as the cassettes are used they tend to wear, bend, and warp, making the precise positioning of the wafers all the more questionable. For example, various hot dip processes tend to severely wrap cassettes.
Another problem with cassettes is that the wafers fit rather loosely within the slots. Since the base portions of the wafers are not immobilized the distance between the base portions of adjacent wafers can vary, compounding the difficulty of automated removal of the wafers.